State of the Art
The discontinuous coating of metal bodies with a zinc-aluminum alloy is disclosed in the '049 application. Also as disclosed therein, drawbacks were encountered whereby uneven coatings or bare spots were obtained. Prior to the method disclosed therein, acceptable coatings were obtainable only with difficulty and by complicated, time consuming procedures.
The '049 application discloses a very good solution to such drawbacks, essentially comprising a method whereby the metal bodies to be discontinuously coated are preferably electrolessly pre-coated with a metallic pre-coating, before the immersion in the zinc-aluminum molten bath. The pre-coating is preferably a metal chosen from the group consisting of copper and nickel. Cobalt could be used, but is not preferred for a number of reasons, including cost and toxicity. The pre-coating forms a very thin coating, permitting a good adhesion of the zinc-aluminum alloy.
However, even if a pre-coating is used, the final layer of zinc-aluminum alloy may present a number of adhesion, compactness and appearance defects, attributed to the formation of metal oxides during air drying after the pre-coating and prior to the immersion of the pre-coated metal body in the Zn--Al bath. Such oxides prevent a proper formation of the final coating. This outer oxidation layer, particularly for baths containing 0.1-25% wt. % Al, is a physical barrier against the inter-action or reaction of the pre-coating metal and the Al in the bath.
Attempts were made to eliminate such superficial oxidation through mechanical polishing with emery papers aided by a final treatment with alumina impregnated cloths. Another treatment utilized was a surface activation by pickling in diluted hydrochloric acid, followed by water rinsing and drying. Neither of these approaches yielded consistently satisfactory results.
It is an object of the present invention to avoid those drawbacks, through a surface conversion treatment resulting in a compact, continuous and glossy coating.